1. Is this for Imperials or chaos? Because as far as chaos go, that is exactly what they do, the Night Lords series show them doing this quite a bit. If Imperials, you could easily make up a fluff reason, such as their fortress monastery was destroyed or is just completely cut off from any nearby Ad Mech support, that the chapter is on a penance crusade for something they did and have had all support from foundries or the Mechanicum cut off until the penance is over. Stuff like that.
2. Some just don't want to. A lot of them just don't care about the Imperium anymore, they have actually got over it and are far too concerned with fighting current grudges against other warbands in the Eye. Also just wanting to consolidate their own power, some have made worlds their own or become independent brokers and hubs for other warbands, and again have no interest in going to potentially die at the hands of the Imperium, as powerful as they would be united, many would die, and the current state of Chaos marines is very much selfish, self-serving, ambitious and with a high level of self preservation in a lot of cases, they just aren't like the loyalists they used to be or the Imperials, ready to lay down their lives without hesitation. And of course many, like the Primarchs are just playing the Great Game now. That being said. Abaddon has secured the service of all the remaining traitor Primarchs for the 13th Black Crusade, which is going to be the most devastating one yet and will in all likelihood end the Imperium or cripple it so badly that it can never recover. Though that is of course prime time for the missing loyalist Primarchs to return.
3. The Wolf Time. There is no set time, it's the end time, the final moments of the Imperium. As above, that could well be the 13th Black Crusade. It implies that even if Russ intends to return for that, it will be the end regardless. The Lion will also likely be released by the Watchers then, as they are holding him until he's needed most. Cue the Lion and the Wolf King entering Commorragh (where I think he is anyway) to jailbreak the Khan out. Have Vulkan and Corax come out of hiding, have some bullshit deus ex machina to resurrect Guilliman, have Dorn(sans hands(and I still says he's dead anyway)) magically appear and we got some big heavy hitters arrayed against Chaos. Sanguinius is proper fucked though. Won't see him, unless of course James Swallow gets to write stuff for it, in which case the Sanguinor will probably become Sanguinius himself once the others return. This is all moot though as G-Dubs will never write the end times for 40k.